9 Post Translational Processing Of Proteins

Question 1

What is protein targeting?

Answer 1

Biological mechanism by which proteins are transported to appropriate destination in or out of the cell

Question 2

What happens with a protein that’s destined for the cytosol?

Answer 2

1. Proteins synthesised on free ribosomes
2. Completed polypeptide dissociates from ribosome
3. Remains in cell in cytosol

Question 3

What happens to proteins post-translation? (2)

Answer 3

1) proteolytic cleavage - peptide bonds broken to remove part of protein
2) chemical modification - addition of functional groups to AA residues

Question 4

What happens to proteins that are targeted for cell organelles?

Answer 4

1. Contain a signal intrinsic to protein
2. Receptor recognising the signal directing it to the correct membrane
3. A translocation machinery - transporting it there
4. Energy to transfer the protein to its new place

Question 5

How do proteins make their way to peroxisomes?

Answer 5

1. Signal - Peroxisome Targeting Sequence (PTS)
   - Serine, Lysine, Leucine (SKL) near C terminus
2. Receptor
   - Pex5 binds to cargo protein
3. Translocation machinery
   - 13 pex proteins make up a transport channel across peroxisomal membrane
   - binds to pex 5-cargo complex
4. Energy
   - ATP hydrolysis allowing recycling of PTS receptor (pex 5)

Question 6

What happens when protein targeting to the peroxisome goes wrong?

Answer 6

Zellweger syndrome

• reduction or absence of functional peroxisomes
• tissues and cells accumulate very long chain fatty acids that are normally degraded
• reduction in CNS myelin

Question 7

How to proteins make their way out of the cell (Secretion)?

Answer 7

1. Signal
   - at N terminus of secretory proteins
   - central region rich in hydrophobic residues
   - can form alpha helix so able to cross lipid bilayer easily
2. Signal recognition particle (SRP)
   - 6 proteins and 1 short piece of RNA recognises signal and ribosome, they attach
3. Translocation across ER membrane
   - GTP=>GDP +Pi so SRP recycled and translocon opens
   - polypeptide moves in and signal peptidase cleaves signal sequence
   - ribosome dissociates from ER and folded protein forms
4. Budding and fusion of ER to cis golgi
5. Cisternal progression to trans golgi
6. Secretory vesicle out of cell

Question 8

How do proteins make their way into the ER membrane?

Answer 8

1. Signal
   - at N terminus of secretory proteins
   - central region rich in hydrophobic residues
   - can form alpha helix so able to cross lipid bilayer easily
2. Signal recognition particle (SRP)
   - 6 proteins and 1 short piece of RNA recognises signal and ribosome, they attach
3. Translocation across ER membrane
   - GTP=>GDP +Pi so SRP recycled and translocon opens
   - polypeptide moves in and signal peptidase cleaves signal sequence
   - stop transfer anchor sequence reached - pauses transfer of peptide across membrane and holds in place

Question 9

What is the functions of the endoplasmic reticulum?

Answer 9

1. Insertion of proteins into membranes
2. Specific proteolytic cleavage
3. Glycosylation
   - addition of sugar residues - N linked
   - folding purposes, stability, cell cell recognition
4. Disulphide bonds
   - for extracellular proteins going to hostile areas (stability)
   - protein disulphide isomerase
5. Proper folding of proteins
   - proteins trapped in misfolded conformation, contains mutation resulting in misfoding, incorrectly associated with other subunits
   - done by ER chaperones
6. Assembly of multisubunit proteins
7. Hydroxylation of selected lys and pro residues (next lecture)

Question 10

How does an ER chaperone attempt to correct protein misfolding?

Answer 10

Retain unfolded in ER, act as sensors to monitor extent of protein misfolding

Question 11

What happens if misfolding cannot be corrected by chaperones?

Answer 11

1. Protein returned to cytosol for degradation (insulin receptor for diabetes)
2. Protein accumulate to high levels resulting in disease (alpha1 anti-trypsin causing liver failure)

Question 12

What modifications occur in golgi apparatus?

Answer 12

1. Sorting into lysosome, plasma membrane, secretory vesicle
2. Phosphorylation of oligosaccharides on lysosomal proteins
3. Removal and addition of bits and bobs
4. Sulfation of tyrosines and carbohydrates

Question 13

What is o-linked glycosylation?

Answer 13

Occurs in golgi
Attachment of sugar to side chain OH of serine, threonine
Proteoglycans (components of ground substance in connective tissue and mucus secretions)

Question 14

How is preproinsulin processed?

Answer 14

1. Translated from mRNA - preproinsulin
2. Entry into endoplasmic reticulum and removal of signal peptide - proinsulin
3. Disulphide bond formation in ER between three pairs of cysteines
4. Leaves ER and Enters golgi, beta chain removed - insulin

Question 15

Why can proteolytic processing yield different products? What is the advantage of this?

Answer 15

The initial precursor molecules can give rise to a whole range of functional proteins!

So depending on the cell and enzymes available to cut, different proteins are formed from the same precursor.

Conservative :)